Understanding Lead-Free Alloys by Karl Seelig
Understanding Lead-Free Alloys By Karl Seelig
One result of the push to find lead-free solder alternatives is that there are now many options available to the board assembler. Much development, patterning and research has gone into finding viable solutions for those who want to eliminate lead from their process. However, each of these alloys is different in significant ways and background information is necessary. January 1998 The Alternatives: Many of the new products are rich in tin with a variety of At first glance, the bismuth-bearing alloys appear to offer other elements added to enhance different high tensile strength, but in peel strength tests they are characteristics. The most basic solders are binary prone to failure due to poor fatigue resistance. The alloys that have been used for years in non-electronic same can be said for indium-based lead-free applications. Tin-lead has been a suitable standard alternatives. alloy for many years simply because it meets most of Tin/silver as a 96/4 alloy is fairly common and the requirements for electronics assembly. However, has a long history in the hybrid circuit industry. with the urgent need for controlled recycling and Unfortunately, with a spike temperature of 260°C, this reduction of hazardous waste related to finished alloy’s melting point is too high for many surface mount products, the greater use of lead-free solders has led to applications. the discovery of some of their noteworthy characteristics. For example, high joint strength, better Ternary tin/copper/silver alloys comprise a family fatigue resistance, improved high temperature life and of lead-free alternatives that show high promise. harder solder joints are among the features seen in Sources of supply are sufficient and the alloys exhibit some of the newer materials. Nevertheless, it is good wetting, fatigue resistance and good overall joint important to remember that these benefits are very strength. One form * is doped with antimony and much dependent on the specific alloy. Not all lead-frees exhibits an advantage: It does not grow intermetallics are created equally and each should be thoroughly with copper when soaked at 125°C (see chart). investigated before implementation into production. Antimony has been known to stop gray tin transformation at low temperatures and historically, the military required As a general rule, most lead-free solders melt at 0.2 to 0.5 percent antimony to enhance the thermal temperatures higher than those of tin-lead. The cycling properties of tin-lead alloys. exceptions are alloys containing indium or bismuth, which tend to lower the melting point. The main Ternary (with antimony) Alloy on Various Substrates “problem” with indium is its cost, with prices currently in Rate of Intermetallic Growth At 125º C the area of $150/lb. With approximately only 200 tons 10 of indium produced globally each year, the supply is 9 8 quite easily depleted and prone to drastic price 7 6 fluctuation. Typically, choosing an indium-based lead- 5 4 free alloy makes the most sense with temperature- 3 sensitive components that do not require high joint 2 Thickness (µm) 1 strength and where the end product will not be 0 50 100 150 200 250 subjected to harsh or high stress environments. Time (days) As for bismuth-based lead-free alloys, a lower Copper Brass Nickel Alloy42 melting temperature than that of tin-lead is offered together with a cost similar to that of tin (in the area of $3/lb.) Unfortunately, bismuth in soldering alloys tends However, the issue of the toxicity of antimony arises. As to create embrittlement, and if a bismuth-based alloy with most metals, salts, oxides and organo-metallic picks up any lead, the melting temperature will drop compounds of antimony are typically the most toxic again, causing joint embrittlement. forms of the element.
1 -2 Understanding Lead-Free Alloys They, however, do not form in standard soldering processes. Antimony has been approved for use in potable water systems as well as in food containers. In pewter tableware, which is commonly used in the preparation of food, antimony is often found at levels of 7 to 9 percent, and copper at levels of 1 to 3 percent. Further the antimony-doped alloy has been environmentally tested to confirm that it will not leach silver or copper into ground water. The accepted reason is that the two elements tied in the tin and antimony reduce their solubility. Lead-free solders are available in paste, bar and cored-wire form, although the latter’s manufacturability remains an issue for alloys that contain high levels of bismuth or indium, which tend to inhibit their drawing ability. Lead-free solders typically need higher soldering temperatures and successful implementation involves resolution of several critical issues.
· The physical (thermal and mechanical) needs of the joint. · The temperature requirements of the components to be soldered. Most parts and board materials can take up to a 240°C spike, but some specialized exceptions remain.
· The field environment (temperature, acceleration and shear) of the end product. · The phase-in period for a complete transition to a lead-free regime. (How long will tin-lead components and tin-coated PWB’s be used during the transition period?)
· Repairs on in-field products. This may militate against using bismuth or indium-based lead-free alloys for assembly since lead significantly affects their melting points. With a great number of ongoing and completed studies on lead-free soldering alloys available to the end-user, there is a wealth of information entering the public domain. Preliminary results suggest that the tin/silver/copper-based alternatives offer superior long- term reliability and joint strength for most applications and that antimony as an additive brings good high Karl Seelig is V.P. of Technology at AIM in Cranston, RI temperature soak characteristics.¤ E-mail: [email protected] This article may also be found in the November ’97 issue *CASTIN of SMT
2 -2 Understanding Lead-Free Alloys